Open-end rotor spinning machines have been known for a long time and generally consist of a large number of similar workstations arranged next to one another in a row. In addition, they have a central control mechanism as well as a negative pressure system particular to the spinning machine Each of these workstations has a spinning and a winding device, on which a fiber band (commonly referred to as a sliver) presented in a spinning can is spun to form a yarn and is wound to form a cross-wound bobbin.
The fiber band is fed by means of a fiber band feed cylinder to an opening cylinder which, with its clothing, opens the fiber band into individual fibers and transports them to a fiber guide channel of the spinning box. The fiber transport is assisted by negative pressure existing in the rotor housing, which produces an air stream in the fiber guide channel, releases the fibers from the opening clothing and conveys them in a targeted manner into the spinning rotor by means of a so-called channel plate adapter. Owing to the centrifugal acceleration of the spinning rotor, the fibers slide into a collecting groove of the spinning rotor, are collected there, are drawn off axially by a withdrawal nozzle in the rotational axis of the rotor and thus rotated to form a yarn that is wound onto a cross-wound bobbin.
After a stoppage of the rotor spinning machine or after a yarn break, the spinning process has to be resumed. For reconnecting the trailing end of yarn previously formed to fibers in the spinning rotor, called piecing, the trailing yarn end is guided counter to the yarn withdrawal direction into the yarn withdrawal tube of the spinning chamber and suctioned in by the negative pressure in the rotor housing. The cross-wound bobbin is pivoted down, in other words brought into contact with a bobbin drive roller again and as soon as the yarn end reaches the collecting groove of the spinning rotor or the fiber ring located therein, the yarn end breaks open the fiber ring and the spinning process can be continued.
An open-end rotor spinning machine, which, in the suction line for producing the negative spinning pressure in the rotor housing, has a valve with rapid venting, is disclosed by European Patent Publication EP 0 529 312 B1. The suction of the rotor housing is interrupted before piecing by means of the valve. As a result, the fibers are not suctioned, as usual during the spinning process, through the fiber guide channel, but through the suction opening of the opening roller. With the introduction of the yarn end into the spinning rotor and by opening the valve, the rotor housing again has negative pressure applied. The suction through the suction opening of the opening roller is ended so as to be synchronized with this with respect to time. A more uniform yarn piecing is to be produced by this deflection of the fiber stream during the piecing process.
German Patent Publication DE 10 2006 037 849 A1 discloses an open-end spinning machine and a method for controlling an open-end spinning machine during a setting process when repiecing a yarn or after a yarn break. Proceeding from the fact that during repiecing after the closing of the housing, the rotor interior is cleaned by means of so-called rinsing air of subsequently introduced impurities that have accumulated there, that each spinning station requires its own rinsing air guide for this step and that the automatic piecing mechanisms or maintenance mechanisms additionally must have corresponding connections to introduce the rinsing air, a repiecing of the yarn is to be made possible with reduced outlay with respect to the device and method.
For this purpose, according to German Patent Publication DE 10 2006 037 849 A1, upon a stoppage of the fiber band feed or after a predetermined time period, the negative pressure present in the housing is immediately interrupted. This results in the prevention of the fiber band end being drawn by the air, which continues to flow in, to the opening mechanism and prevents the occurrence of an undesired suctioning in of impurities before the housing of the spinning station is closed. In addition, energy can be saved in this manner as a considerable quantity of additional air is suctioned into the machine through the opened housing when negative pressure is present, which would in turn lead to a pressure increase within the pressure systems.
The drawback in the methods and devices according to the prior art, is, however, that the rotor housings of the individual workstations permanently have negative pressure applied up until the piecing process or the cleaning process connected therewith.
This has a negative effect quite particularly after a restart of the machine or run up after a power failure. As all the workstations have negative pressure applied and additional mechanisms, such as for example, the suction nozzle for grasping the upper yarn or a pneumatic yarn store, which also require negative pressure, are used for piecing, the negative pressure available overall is reduced. Therefore, in a rotor spinning machine with self-sufficiently piecing spinning stations, only a limited number of the spinning stations, e.g., only 12 stations, can repiece simultaneously, as the negative pressure is not sufficient for piecing at further workstations, including the additional mechanisms necessary for this process.